In Patent Documents 1 to 3 below, wet film-forming methods used to produce a hole-injecting layer of an organic electroluminescent device are described.
Patent Document 1 discloses a method used to form a hole-injecting/transporting layer by a spin-coating technique wherein a solution containing a hole-transporting material, polyether with aromatic diamine, and an electron-accepting compound, tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH), in dichloromethane is used.
Patent Document 2 discloses a method used to form a hole-injecting layer by a spin-coating technique, wherein a solution containing polyether with aromatic diamine in 1,2-dichloroethane is used.
Patent Document 3 discloses a method used to form a hole-injecting layer by a spin-coating technique, wherein a solution containing a mixture of 4,4′-bis[N-(m-tolyl)-N-phenylamino]biphenyl and an electron-accepting compound, antimony pentachloride, in 1,2-dichloroethane is used.
In Patent Documents 4 and 5 below, compositions for forming a hole-injecting/transporting layer by an inkjet technique are described.
Patent Document 4 discloses a coating liquid prepared by dispersing copper phthalocyanine or a conductive polymer, polyethylenedioxythiophene (PEDT), and polystyrene sulfonate (PSS) in a mixed solvent of water, lower alcohol and other components.
In Patent Document 5, a coating liquid prepared by dispersing PEDT and PSS in a solvent composed of water, ethanol and dipropylene glycol is disclosed.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-283750    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2000-36390    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-56985    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-106278    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-204114
Compositions for the formation of a hole-injecting/transporting layer that are used in known wet film-forming methods often contain water as solvent. In general, the presence of water would impair the properties of an organic electroluminescent device. Therefore, as much water as possible must be removed from the formed layer when such a water-containing composition is used. It is difficult to eliminate water from the formed hole-injecting/transporting layer. Existing organic electroluminescent devices have deteriorated properties due to the moisture remaining inside the layer. In manufacturing of the organic electroluminescent devices, fluctuations in the remaining moisture amounts among individual organic electroluminescent devices that occur during the film-forming step results in the devices having inconsistent properties.
Most film-forming materials used for producing a hole-injecting layer or a hole-transporting layer of an organic electroluminescent device, such as 4,4′-bis[N-(m-tolyl)-N-phenylamino]biphenyl and polyether with aromatic diamine, have low solubility in commonly used solvents, thus making it difficult to prepare a solution of an appropriate concentration when producing a thin layer of organic material by a wet film-forming method.
To form a highly uniform hole-injecting/transporting layer, it is important that the layer has a high affinity for the substrate. Accordingly, a solvent used to prepare a solution used in a wet film-forming method is required to have not only a capability of dissolution of a hole-injecting/transporting material but also a high affinity for the substrate. However, it has been difficult to prepare a solution that satisfies both these requirements evenly.
When producing an organic electroluminescent device, wherein two or more layers are laminated, by an inkjet film-forming technique, a drying rate of the coating liquid is significantly important for controlling the efficiency of manufacturing processes. For example, the use of a solvent with a higher vapor pressure causes the solvent to evaporate during spraying of the coating liquid from a spray nozzle onto a surface to be coated and clogging in the nozzle to occur, thus making it difficult to form a highly uniform organic layer.